A surface coating is sometimes applied to a recording medium in order to improve its printing properties. For example, the coating can improve the appearance, ink absorption, and/or image smear resistance of the medium.
Surface coatings can be classified into two general categories—glossy coatings and non-glossy (matte or dull) coatings. Glossy coatings are highly desirable, as they are very smooth, and can impart a superior feel and a photograph-like quality to a recorded image. However, it remains a challenge to provide a glossy medium that imparts superior printing properties to the medium (e.g., good ink absorption, good dye-fixing ability, good waterfastness, and/or good resistance to image smear), in addition to superior smoothness and gloss.
Gloss and dye immobilization (i.e., dye-fixing) can sometimes be achieved by incorporating different types of polymeric resins into a coating. For example, a gelatin, a polyvinyl alcohol, a polyolefin resin, polyester resin, polyamide resin, and/or polycarbonate resin can be used to produce glossiness, while a cationic polymer (e.g., polyvinylpyrrolidone) can be used to promote the surface immobilization of an anionic dye. However, inks applied to resin-coated recording media dry relatively slowly, and often have an undesirable tendency to smear and rub off. While some pigments such as certain treated kaolin clays or treated calcium carbonates can immobilize dyes, the overall absorptivity and rate of absorption are often compromised.
Using a metal oxide pigment such as silica or alumina can be advantageous in that they have good absorptivity and also can produce an excellent coating. Alumina is particularly advantageous in that its particles naturally have a cationic surface (i.e., a positive zeta potential). Since the vast majority of ink dyes are anionic in nature, the cationic surface of alumina imparts superior dye immobilizing properties to coatings derived therefrom. Moreover, alumina also imparts good ink absorption, good waterfastness, and good image smear resistance, in addition to superior gloss, smoothness, and brightness, to the coating.
Despite its advantages, the use of alumina presents significant challenges in the recording medium coating industry in that alumina is very difficult to process. Unlike silica, which is typically amorphous, alumina is crystalline, and can exist in various crystalline phases, for example, alpha, or the transitional phases, for example, gamma, delta, and theta phases. In addition, long drying times are typically required in recording medium coating which utilize low solids alumina dispersions, making the overall coating process costly and inefficient. Moreover, some forms of alumina require a relatively high binder ratio (about 3:1 pigment to binder ratio). The high binder demand of alumina restricts the ratio of alumina particles (relative to binder) that can be achieved in the coating, sacrificing desirable properties that could otherwise be imparted to the coating by the alumina particles (e.g., drying time, dye immobilization, waterfastness, image quality, and the like). As such, the overall quality of the recording medium can be limited.
Poor colloidal stability of alumina also seriously limits the solids content that can be attained in coating compositions used to make the recording media, thereby placing an upper limit on coater productivity (throughput), as drier demand can be excessive in order to adequately dry the coating on the substrate. In a commercial setting, such coating compositions are produced from an initial alumina dispersion. The initial dispersion is often manufactured in a separate facility and shipped to the end user. Typically, the end user processes the initial dispersion into a coating composition, which is normally applied to a substrate shortly after its production.
As dispersions with higher alumina solids content have a greater tendency to gel or separate (i.e., the solid settles out of the dispersion), low solids initial dispersions are used. As such, the overall quality of recording media is limited by the low alumina solids content (e.g., in terms of porosity, dye immobilization, image quality, or the like).
Accordingly there remains a need for an improved recording medium comprising alumina particles, desirably having a low binder demand and high porosity, as well as an alumina-based coating composition and a method of producing such a composition and recording medium. The present invention provides such a recording medium, coating composition, and methods of making them. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.